Dark galaxy spotted with the help of globular clusters

Dark galaxy spotted with the help of globular clusters

When you look at the night sky, all the light you see is what astronomers call ordinary matter. It represents only 15% of the matter (mass) in the universe. A mysterious substance called dark matter makes up the other 85%. So when you look at a galaxy through a telescope, all the light you see is ordinary matter. But is it possible to have a galaxy made out of dark matter? On June 20, 2025, the University of Innsbruck in Austria said researchers have discovered a dark-matter galaxy candidate in a cluster of galaxies some 240 million light-years away.

The astronomers found the candidate dark galaxy – or CDG-2 – in the Perseus cluster, which contains thousands of galaxies in the direction of the constellation Perseus the Hero. Because the newly discovered galaxy is dark (and, in fact, one of the faintest known galaxies), astronomers only suspected its presence due to globular clusters.

It’s common for spiral galaxies, such as our own Milky Way, to have massive balls of stars orbiting in a halo around the galaxy. These huge spheres of stars are globular clusters. The astronomers used data from the Hubble Space Telescope and Euclid survey to first pinpoint four globular clusters and then confirm an extremely faint associated glow.

Lead author Dayi (David) Li of the University of Toronto said:

Globular clusters don’t clump together unless there’s a substantial gravitational force – such as that from a galaxy – to hold them in place.

Co-author Francine Marleau of the University of Innsbruck added:

CDG-2 is the first galaxy identified solely by its globular clusters, and it is among the faintest known galaxies surrounded by such clusters.

The researchers published their peer-reviewed study in The Astrophysical Journal Letters on June 16, 2025.

A galaxy made mostly of dark matter

The researchers estimate that up to 99.99% of this galaxy’s mass could be dark matter. The paper said that some 17% to 33% of the visible light from the galaxy comes from its globular clusters. That would make it the galaxy with the highest proportion of light coming from its globular clusters. And, if accurate, CDG-2 might be one of the most dark-matter dominated galaxies yet discovered. Li said:

This discovery provides clear evidence that galaxies can form within pure dark matter halos, where visible stars are only in dense, old star clusters. Our work opens a new window into understanding galaxy formation and dark matter.

Bottom line: Researchers have discovered what they believe is a dark galaxy, made almost entirely of dark matter. They spotted it thanks to the visible light of globular clusters surrounding it.

Source: Candidate Dark Galaxy-2: Validation and Analysis of an Almost Dark Galaxy in the Perseus Cluster

Via University of Innsbruck

Via University of Toronto

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